1. Field of the Invention
This invention pertains in general to ion conductors and to processes for the synthesis of ion conducting solid electrolytes. In particular, the invention relates to the use of nanoscale powders for the preparation of nanostructured oxygen ion conducting electrolytes.
2. Description of the Prior Art
Solid electrolytes are materials through which ion species can migrate with low energy barriers. Table 1 outlines some examples of ion-conducting structures, representative materials, and the ions conducted. These materials are of critical commercial importance to electrochemical devices, components and processes. Illustrative applications include sensors, batteries, fuel cells, ion pumps, membrane reactors, catalysis, and metallurgy.
TABLE 1 ______________________________________ REPRESENTATIVE MATERIALS ION CONDUCTED ______________________________________ Stabilized ZrO.sub.2 System, Stabilized Bi.sub.2 O.sub.3 O.sup.2- System, Ceria, Perovskites Beta-Alumina, NASICON Systems Na.sup.+ AgI, RbAg.sub.4 I.sub.5 Ag.sup.+ Rb.sub.4 Cu.sub.16 I.sub.7 Cl.sub.13 Cu.sup.+ Li.sub.3 N, Li.sub.2 S--SiS.sub.2 --Li.sub.3 PO.sub.4 System, Li.sup.+ Polymer Systems, LISICON Systems ______________________________________
As a specific example, stabilized zirconia is a known conductor of oxygen ions. Accordingly, its properties are utilized in various fields of technology, such as in oxygen sensors for fuel-air ratio optimization of automobiles and furnaces, in oxygen pumps for solid state oxygen separation, in solid-oxide fuel cells for noiseless and clean power generation from chemical energy, and in catalytic membrane reactors.
The oxygen-ion conduction properties of stabilized zirconia used in a typical oxygen sensor are well understood based on electrochemical-cell theory. When placed between two compartments containing a reference gas and an analyte oxygen gas at different partial pressures, stabilized zirconia functions both as a partition between the two compartments and as an electrochemical-cell electrolyte. Under ideal conditions, the open-circuit emf (E.sub.0) of the cell is given by the known Nernst equation: ##EQU1## where T is the absolute temperature of the cell; P.sub.O2Ref and P.sub.O2 are the partial pressures of oxygen in the reference and analyte compartments, respectively; R is the universal gas constant; and F is Faraday's number.
According to this equation, any difference in partial pressure of the oxygen across the two faces of the oxygen-conducting electrolyte generates an electromotive force that depends on the temperature and partial-pressure ratio of the oxygen in the two compartments of the system. In order to generate Nernstian response in sufficiently short times, the temperature of stabilized ZrO.sub.2 needs to be high (above 700.degree. C.), which results in relatively high power requirements and in increased equipment mass and size, need for insulation, and attendant sealing problems. These considerations often produce unsatisfactory performance or affect the commercial viability of products based on stabilized ZrO.sub.2 technology.
The inherent reasons for the high-temperature requirement and the corresponding performance problems of present-day oxygen ion conducting electrolyte based devices can be traced to the reaction mechanism of the cell and the microstructure of the sites where the reaction occurs. Referring to FIG. 1, a schematic drawing of a ZrO.sub.2 sensor cell 10 is illustrated, where the stabilized zirconia is modeled as a solid electrolyte membrane 12 between a first compartment 14, containing a reference oxygen atmosphere at a predetermined partial pressure P.sub.O2Ref, and another compartment 16 containing an analyte gas with oxygen at a different partial pressure P.sub.O2. The two sides of the stabilized zirconia non-porous solid electrolyte 12 are coupled through an external circuit connecting an anode 18 and a cathode 20 made of porous metal, such as silver. The anode 18 is the cell electrode at which chemical oxidation occurs and the electrons released by the oxidation reaction flow from it through the external circuit to the cathode. The cathode 20 is the cell electrode at which chemical reduction occurs. The cell electrolyte 12 completes the electrical circuit of the system by allowing a flow of negative ions O.sup.2- between the two electrodes. A voltmeter 22 is provided to measure the emf created by the redox reactions occurring at the interfaces of the electrolyte with the two oxygen atmospheres.
Thus, the key redox reaction of the cell occurs at the points where the metal electrode, the electrolyte and the gas meet (illustrated in the inset of FIG. 1 as the "triple point" 24). At each such site on the surface of the electrolyte 12, the redox reaction is as follows: EQU O.sub.2 (gas)+4e.sup.- .fwdarw.20.sup.2-. (2)
Since the reaction and the electrochemical performance of the sensor depend on the redox kinetics, the cell's performance is a strong function of the concentration of triple points. In other words, an electrode/electrolyte/electrode cell with as many triple points as possible is highly desirable see Madou, Marc and M. Morrison, Chemical Sensing with Solid State Devices, Academic Press, Boston (1989)!. In the case of an oxygen cell with a ZrO2 solid membrane and silver electrodes, this requirement corresponds to maximizing the triple points on each side of the PO2.Ag'/ZrO2/Ag".PO2Ref system.
Another cause of poor performance of oxygen-sensor cells can be explained with the help of complex-impedance analysis. Referring to FIGS. 2a and 2b, a complex impedance diagram for a ZrO.sub.2 sensor is shown, where the impedances of the bulk, grain boundary and electrode are illustrated in series to reflect their contribution to the ionic conduction at each triple point. It has been shown that the conductive performance of electrolytes at temperatures below 500.degree. C. is controlled by the grain boundary contribution to the overall impedance. Thus, for significant improvements of the conductivity at low temperatures, it is necessary to significantly minimize the grain-boundary (interface) resistance.
In summary, oxygen ion conducting devices based on stabilized-zirconia electrolyte have two problems that can be traced to material limitations. First, the electrolytes have high impedance; second, the concentration of triple points is relatively low. These problems are common to solid oxygen-conducting electrolytes in particular and solid electrolytes in general, and any improvement in these material characteristics would constitute a significant technological step forward. The present invention provides a novel approach that greatly improves these aspects of ion conducting solid electrolytes.